This study is designed to examine the time course of stress- induced analgesia (SIA) and the elevations in mood and plasma endocrine levels produced in man by strenuous physical exercise; to determine the relative contributions of endogenous opioid and non-opioid factors to these phenomena; to look for significant relationships, if any, among SIA, mood elevation and various plasma endocrine levels; and to test the possibility that SIA is a non-specific phenomenon - i.e., that stress elevates not only pain thresholds but other sensory thresholds as well - e.g., auditory thresholds. The study will be carries out with the aid of a treadmill in 96 subjects. An indwelling catheter will be inserted into an antecubital vein of each subject for drawing blood samples and injecting naloxone or placebo. Tests for thermal discriminability, ischemic pain sensitivity, auditory discriminability and a mood questionnaire will be administered before, during and after various intensities of theadmill stress - under naloxone or saline conditions. Blood samples taken before, during and after the treadmill stress will be analyzed for plasma ACTH, beta- endorphin - ir (beta-endorphin like immunoreactivity), prolactin, epinephrine and norepinephrine. Test results will be subjected to appropriate data analysis; additionally the thermal pain test data will be subject to sensory decision theory analysis. Instances in which the pain of injury is dulled during athletic events, accidents, or the stress of battle are matters of common knowledge, sometimes personal experience, and often the subjects of anecdotal accounts. Despite the common occurence of this phenomenon it has received surprisingly little systematic study in humans, although SIA is an area of active investigation in laboratory animals. The proposed investigation is an experimental attempt to examine the phenomenon in man, test its specificity and determine its relationship to endogenous opioid pain regulatory mechanisms. In examining the mood-elevating effects of exercise the study also provides an opportunity to investigate a possible role of endogenous opioids in mood regulation.